1. Technical Field
This invention relates to a motor having a fluid dynamic bearing and a disk drive for rotating a recording disk using the motor.
2. Description of the Related Art
In recent years, the storage capacity of the recording disk drive including the hard disk drive has remarkably increased while the size thereof has been decreased. With the increase in storage capacity, the motor (spindle motor) used with the recording disk drive to rotate the recording disk is required to be further increased in rotation accuracy and decreased in size. Further, the recording disk drive mounted on a portable device is liable to be exposed to external vibrations and, as compared with the conventional motor, required to remarkably reduce the RRO (repeatable run out: the displacement of the synchronous component of the rotor of the motor in operation) and the pure tone (the pure tone noise generated by resonance of vibration of motor) generated by resonance. In view of this, the motor in this field employs a bearing using a fluid dynamic pressure to improve the quietude and the rotation accuracy. For example, a fluid dynamic bearing having a plurality of grooves is known. The number of the grooves is considered to be preferably a multiple of three from the viewpoint of the accuracy of fabrication jigs and symmetry, and 3, 6, 9 or 12 grooves are employed in actual applications.
On the other hand, a DC brushless motor is used as a spindle motor. This DC brushless motor includes, for example, a rotor magnet having a plurality of diametrically magnetized poles and a stator having a plurality of teeth (magnetic pole teeth) arranged in diametrically opposed relation to the rotor magnet. In order to secure a high duty factor, the stator coil is excited by a three-phase voltage applied thereto. For the purpose of reducing vibrations, noises and cogging, a DC brushless motor is known in which the rotor magnet has 8 poles and the stator has 6 teeth with 3, 5, 7 or 11 grooves of the fluid dynamic bearing. Depending on the number of the poles and slots of the motor actually selected, however, vibrations are often caused regardless of the number of the grooves. Also, even for the number of the grooves which is considered to hardly cause vibrations, the synergy superimposition resonance of vibrations of the motor and the bearing causes large vibrations.